Long-term AZT therapy can cause myopathy, as demonstrated by light and electron microscopy of muscle biopsies. 31P NMR spectroscopy is an established method for studying myopathies, and could be helpful in monitoring muscle energy metabolism, during AZT therapy. We used 31P NMR spectroscopy to study 20 HIV positive patients and 19 age-matched volunteers. HIV-positive patients undergoing AZT therapy showed a significantly greater degree of phosphocreatine (PCr) depletion for a given work load, compared to controls. However, HIV patients that were not treated with AZT did not have a significant PCr depletion, compared to controls. Our findings support previous morphological and molecular finding that myopathy observed in AZT-treated patients is due to mitochondrial dysfunction. They also demonstrate that 31P NMR spectroscopy is as sensitive as electron microscopy in detecting mitochondrial abnormalities in AZT treated patients. We conclude that 31P NMR spectroscopy is a useful non-invasive approach for monitoring HIV- infected patients during AZT therapy.